@Sunny Karen sounds like a great plan and very workable. A couple of thoughts:
1) I would probably opt for 30 year and paying more than you have to for the emergency flexibility you mention. It is a long time till kids college. What if you lose the W2. You want this to be truly self-sustaining, yes?
2) Not sure what the loan structure is, but you may want to make sure you can separate the properties easily if you want to sell one.
3) Consider putting down just slightly less (in combo with above?) and keeping an emergency fund in a liquid high yield account...you mentioned not being able to cover a big emergency.
4) I would look into life insurance/upping disability policies for you and your husband if you don't have already timed around the mortgage. Terms are incredibly cheap and its an added layer of protection
5) Consider channeling some of the positive cash flow from these into 529 plans for the kids. A good accountant could let you know how much of a benefit that could be. If you find yourself wanting to sell early each of you is able to fund those 5 years in advance at 15K a year each. So that could be a powerful tool
6) Consider the future potential of condos in the duplexes...not sure of the way your market works, but the world is kinda going that way. The ability to condo would allow you even more flexibility to sell pieces. You could even consider doing the docs now, etc.
7) keep an open mind about the tactics of selling/exchanging early. 15-18 years is a long time, and you will be near the end of the baby boomer bubble. If you find yourself with a great deal of equity, you may want to capitalize on it. Strategy and goal can remain, but tactics can shift.